Close vascularization implant material

ABSTRACT

A device for implantation in a host having a material at an interface between the host and the device, said material having a conformation which results in growth of vascular structures by the host close to the interface.

This is a continuation of application Ser. No. 08/210,068, filed Mar.17, 1994; which is a continuation of application Ser. No. 07/933,871,filed Aug. 21, 1992 (abandoned); which is a continuation of applicationSer. No. 07/735,401, filed Jul. 24, 1991 (abandoned), which is acontinuation-in-part of application Ser. No. 07/606,791, filed Oct. 31,1990 (abandoned).

BACKGROUND OF THE INVENTION

The present invention relates to material implanted in a host. Moreparticularly, the present invention relates to material that promotesthe formation of vascular structures at the interface between at least aportion of the implanted material and the host.

For a variety of applications, ranging from research to therapeutic, itmay be desirable to implant an article or device within soft tissue.Such implants can include indwelling catheters, indwelling sensors, anddevices for holding tissue that are implanted in vivo.

If the implanted device is utilized to hold tissue, in a variety of suchapplications it is necessary to isolate the implanted tissue from theimmune response of the host (immunoisolation). For example, this iscritical when the implanted tissues are xenografts, i.e., graft cellsfrom donors of another species, or allografts, i.e., cells from the samespecies but having a different genetic make-up. A failure to properlyisolate such tissue will result in an invasion from host cells or hostimmunogenic factors rejecting the implant cells. In certain otherapplications, such as autografts, i.e., cells previously isolated fromthe tissue of the patient to be implanted, it is necessary to isolatethe implanted tissues from the host, not because the cells would berejected, but because the cells may contain retroviral vectors whichotherwise might present a risk to the patient. Accordingly, it may benecessary for such cells to be enclosed within a structure that preventsthe passage of cells therethrough.

In certain other implant applications it may be desirable to provide azone or structure that is selectively impermeable for moleculardiffusion as in certain forms of cellular implants that could berejected by humoral factors, or non-permeable for non-transportfunctions, such as providing a surface for transcutaneous catheters.

When biomaterials are implanted, the host inflammatory cells(macrophages, giant calls, and fibroblasts) produce an inflammatoryresponse called a foreign body response. This response invariablyresults in a zone of nonvascular tissue that surrounds the implantedmaterial. The foreign body response is the body's attempt to remove orisolate the foreign entity (Anderson, J. M., "Inflammatory Response toImpants", Trans. Am. Soc. Artif. Interm. Ograns, Vol. XXXIV:101-107(1988)).

During the foreign body response macrophages from the host attempt toingest the foreign body. In some cases, the macrophages coalesce to formmultinuecleated giant cells. The implant may lead to the formation offibroblast layers of increased thickness and density as the hostattempts to isolate the foreign body. This creates a fibrous capsule ofcells and collagen.

Referring to FIG. 1, a micrograph (1(a)) and a drawing (1(b)) areprovided to illustrate a classical tissue response to an implantedforeign body. FIG. 1 represents a typical histological section takenthrough a tissue block removed after approximately three weeks from adorsal subcutaneous implant in a Sprague-Dawley rat. As illustrated, theimplant 10 is surrounded by a foreign body capsule 12 that formsadjacent to the implant. The foreign body capsule 12 typically consistsof three-layers.

As illustrated, the first layer 13 of the foreign body capsule 12includes macrophages 14 and foreign body giant cells 16 at an interface18 between the implant 10 and the tissue. This first layer 13,consisting of the macrophages 14, is generally approximately 5 to about15 microns thick.

The next, or second layer 15, of the foreign body capsule 12 includesfibroblasts 20. The fibroblasts 20 are oriented parallel to the surfaceof the implant 10 and embedded in a collagenous matrix includingcollagen fibers that are also oriented parallel with the surface of theimplant. The second layer 15 consisting of the fibroblasts 20 andcollagen fibers is generally approximately 30 to about 200 micronsthick. The first and second layers 13 and 15 of the foreign body capsule12 are usually completely avascular throughout.

At the outlying areas of the foreign body capsule 12, a few vascularstructures 24 begin to appear in the outer regions of the fibroblastsecond zone 15. At a third layer 17, lying approximately 30 to about 200microns away from the surface of the implant 10 is loose connectivetissue that is highly vascular. This layer 17 is amorphous and widelyvaries in thickness depending on the tissue location and time after theimplant.

As illustrated in FIG. 1, the classical foreign body response results inthe implant 10 being surrounded by a foreign body capsule 12 that doesnot include vascular structures near the surface of the implant.

Although the foreign body capsule generated from the foreign bodyresponse is desirable, or at least not detrimental, for certain types ofimplants, such as, for example, silicon breast implants and collagenimplants, the foreign body capsule prevents certain applications andtreatments utilizing such implants. For example, indwelling sensors forapplications such as glucose analysis in diabetics, become occludedafter only a few days due to the foreign body capsule. Indeed, theforeign body capsule becomes so thick that it inhibits the diffusion ofglucose to the membrane surface preventing the sensor from functioning.

Likewise, when pancreatic islets are implanted within a semipermeablemembrane for treatment of diabetes, they usually die within a few daysor weeks. The loss of function of the pancreatic islets is attributed tothe poor diffusion of nutrients to the islets due to the thickness ofthe foreign body capsule. Likewise, other tissues that are implantedwithin the host do not remain viable due to the foreign body capsulethat effectively prevents the transport of nutrients from thecapillaries to cells enclosed within the implanted membrane.

Scharp, in a comprehensive review of the literature aboutimmunoisolation ("Isolation and Transplantation of Islet Tissue" (1984)World J. Surgery 8:143-151) cited 18 papers on islet immunoisolation. Inevery case, the islets failed to function for more than a few weeks, orin 4 studies, several months. In every case but one, the failure wasattributed to fibroblastic overgrowth of the membrane and chamber. Theauthors state that, "If . . . a [membrane] can be constructed to resisthost fibrotic response, then the extravascular diffusion chamberapproach may be useful clinically." They further state that the "primarydisadvantages [of diffusion chambers] relate to the host fibroblasticresponse to the device." This belief is echoed in U.S. Pat. No.4,298,002 which states, "the device . . . remains effective for limitedperiods of time because the body encapsulates the device with fibrousmaterial blocking the passage of insulin, nutrients, and/or wasteproducts."

More recent papers continue to state that device failure is caused bythe foreign body capsule diminution of diffusion. For example,Christenson, Abeischer, McMillan, and Galletti, in "Tissue Reaction toIntraperitoneal Polymer Implants: Species difference and effects ofcorticoid and doxorubicin" ((1989) J. of Biomed. Mat. Res. 23:705-718)stated, "reduction of the tissue reaction around an implant is importantin improving the long-term viability of the encapsulated endocrinetissue and is imperative for any clinical application of this techniquefor implanting endocrine tissue."

Poor viability of tissues has prompted the design of modalities forperiodic replacement of implanted islets through percutaneous catheters(e.g. U.S. Pat. No. 4,378,016) to solve the shortcoming of previousdesigns, i.e., the deterioration of implanted tissues because ofovergrowth by a fibrous capsule.

Additionally, indwelling catheters that have a variety of applications,typically have a high drop-out rate because the site of the catheterentry becomes infected. It is generally believed that this infection iscaused by poor adhesion of the tissues to the catheter surface and poorvascularization of the region around the catheter because of the thickforeign body capsule that forms. Implants have been proposed havingsurfaces designed to increase the adhesion or anchorage of the implantin the host tissue (e.g. European Patent Application No. 0359575 of VonRecum and Campbell). This patent application describes materials withsurface topography designed to provide "improved soft tissue implanthaving a surface texture that optimizes anchorage of the implant to thetissue without causing inflammatory tissue at the implantation site."

In attempting to provide needed nutrients to cells and tissues locatedwithin implanted devices and/or allowing agents generated by the tissuesto enter the host, an almost contradictory concern must be dealt with.For devices that include xenografts or allografts, these tissues must beisolated from the immune system of the host. Therefore, although it maybe desirable to somehow connect the vascular system of the host to thesetissues to provide nutrients and allow a transfer of biological agentsto the host, a contrary concern is to prevent an immune response fromthe host to the tissues. Likewise, with respect to sensors andcatheters, although it may be desirable to create vascularization withrespect to these devices, vascularization into an interior of suchdevices will prevent the devices from functioning satisfactorily.

SUMMARY OF THE INVENTION

This present invention provides an implant material that results inclose vascularization by the host it the interface between the materialand the host into which the material is implanted.

The uses of the material of the present invention include: as a coatingfor indwelling catheters; means for transport of physiological factorsto indwelling sensors; means for transport of drugs from a chamber orcatheter to the tissues of the host; and means for encapsulation ofgrafted cells for treatment of cell and molecular deficiency diseases(immunoisolation).

In an embodiment, the present invention provides an asymmetric materialhaving a first zone that induces close vascularization at the materialhost interface and a second adjacent zone that prevents passage of cellsthrough the zone. The vascularizing zone allows the material to bevascularized while the second zone maintains immunoisolation of theinterior of an implanted device incorporating the invention on itsexterior. The material may consist of a bilayer of zones as described orit may be a gradient of zones. The gradient consists of an outer zonewith a conformation that results in close vascularization. The structureof the material becomes gradually tighter until the material isimpermeable to calls.

In another embodiment, the second adjacent zone is molecular permeablefor selective diffusion. In yet another embodiment the second zone isnon-permeable for use in non-transport functions in devices such asindwelling catheters.

To these ends, the present invention provides an implant having a threedimensional conformation or architectural structure at the hostinterface which allows invasion of the material by mononuclear cells,but prevents the invasion by connective tissue which leads to foreignbody capsule formation within the structure.

Applicants do not fully understand how the close vascularization of thepresent invention occurs. The data presented in the tables and figureswhich follow are consistant with the theory that close vascularizationoccurs if the three dimensional conformation of material interfacing thehost is such that it elicits certain host inflammatory cell behavior.Applicants have observed by light and electron microscopy that closevascularization occurs if in the initial period of implantation, atleast some macrophages entering the material are not activated.Activated macrophage are characterized by cell flattening. Applicantsobserve close vascularization in regions of an implant where themacrophgages that have entered the cavities of the material retain arounded appearance when viewed through light microscopy (˜400×). SeeFIG. 2a. At 3000× (TEM) the rounded macrophage is observed to havesubstantially conformed to the contours of the material. Although thereis a correlation with macrophage shape, it is not clear that macrophagescontrol the observed response. However, it is clear that invasion of thestructure by host cells is required. Although the bulk of the cellsappear to be macrophages, it is possible that other inflammatory cellscontrol the response, therefore we will refer to the invading cells as"inflammatory cells," which include but are not limited to macrophages.

On the other hand foreign body capsule formation occurs when, in theinitial period of implantation, inflammatory cells in contact with theimplant material flatten against those portions of the material whichpresent an area amenable to such flattening behavior by an inflammatorycell (FIG. 6).

In an embodiment, the material that results in formation of closevascular structures is a polymer membrane having an average nominal poresize of approximately 0.6 to about 20 μm, using conventional methods fordetermination of pore size in the trade. Preferably, at leastapproximately 50% of the pores of the membrane have an average size ofapproximately 0.6 to about 20 μm.

The structural elements which provide the three dimensional conformationmay include fibers, strands, globules, cones or rods of amorphous oruniform geometry which are smooth or rough. These elements, hereafterreferred to as "strands," have in general one dimension larger than theother two and the smaller dimensions do not exceed five microns.

In an embodiment, the material consists of strands that define"apertures" formed by a frame of the interconnected strands. Theapertures have an average size of no more than about 20 μm in any butthe longest dimension. The apertures of the material form a framework ofinterconnected apertures, defining "cavities" that are no greater thanan average of about 20 μm in any but the longest dimension. In anembodiment the material has at least some apertures having a sufficientsize to allow at least some vascular structures to be created within thecavities. At least some of these apertures, while allowing vascularstructures to form within the cavities, prevent connective tissue fromforming therein because of size restrictions.

In an embodiment, an asymmetric material is provided having a gradientor layer of varying porosity. At least some of the apertures at thesurface of the material that contacts the host tissue, allowinflammatory cells to enter the cavities. But, due to size restrictions,the apertures do not allow the inflammatory calls to transverse thematerial to the interior of the implant.

In an embodiment of the present invention, an immunoisolation containeris provided that includes a first membrane having cavities and situatedproximal to the host tissue. At least some of the apertures of the firstmembrane have a sufficient size to allow inflammatory cells to enter thecavities and cause at least some vascular structures to contact themembrane. The container includes a second porous membrane, the aperturesof the second membrane being sufficiently small to prevent immune cellsand/or immunogenic factors from entering an interior of the container.The second membrane is situated proximal to graft tissues.

In an embodiment, an indwelling catheter is provided by the presentinvention including a porous membrane and a catheter body, the porousmembrane surrounding at least a portion of the catheter body. At leastsome apertures of the porous membrane have a sufficient size to allowinflammatory cells to enter the cavities and cause at least somevascular structures to form that contact the porous membrane.

In an embodiment, the present invention provides an indwelling sensor.The indwelling sensor comprising a sensor for monitoring a condition oragent in the body and a porous membrane that surrounds at least aportion of the sensor body. At least some of the apertures of themembrane have a sufficient size to allow inflammatory cells to enter thecavities and cause at least some vascular structures to form thatcontact the porous membrane.

The present invention also provides a method for the vascularization ofa surface of an implanted device. The method comprises the steps ofallowing inflammatory cells to enter a first layer of a membranestructure and cause vascular structures to form that contact a surfaceof the first layer of the membrane and preventing the inflammatory cellsfrom entering a second layer of the membrane structure. This embodimentwould be applicable in, for example, a breast prosthesis.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the detailed description of thepresently preferred embodiments and from the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a micrograph that illustrates a classical foreign bodyresponse to an implanted device.

FIG. 1(b) is a drawing illustrating a classical foreign body response toan implanted device.

FIG. 2(a) is a micrograph of an embodiment of the present invention.

FIG. 2(b) is a cross-sectional view of an embodiment of the presentinvention with vascular structures growing at the host-materialinterface.

FIG. 3 illustrates a cross-sectional view of a foreign body capsule in apore of a membrane.

FIGS. 4(a) and (b) are scanning electron micrographs of, respectively, amixed ester of cellulose membrane with a 5 μm pore size and a teflonmembrane with 3 μm pore size.

FIGS. 5(a) and (b) are scanning electron micrographs of, respectively, ateflon membrane with a 5 μm pore size and a polycarbonate with 12 μmpore size.

FIG. 6 illustrates a light micrograph showing the teflon membrane ofFIG. 5(a) implanted for 3 weeks in a subcutaneous dorsal pocket in arat.

FIG. 7 illustrates a cross-sectional view of a bilaminar membranecontaining islets, the membrane having an outer layer that isvascularized and an inner layer that prevents immune rejection.

FIG. 8 illustrates a cross-sectional view of a further embodiment of thepresent invention.

FIG. 9 illustrates an indwelling catheter incorporating the presentinvention.

FIG. 10 illustrates an indwelling sensor incorporating the presentinvention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The present invention provides a material for inducing closevascularization at the interface between the material and host intowhich the material is implanted such that a standard foreign bodycapsule consisting of flattened macrophages, foreign body giant cells,and fibroblasts does not intervene between the vascular structures andthe material. The material can be utilized for various applicationsincluding the creation of a container for implanting tissues to beisolated from the immune system of a host, for surrounding a portion ofa catheter, or surrounding a portion of an indwelling sensor device.

Pursuant to the present invention, the material utilized results in thegrowth of vascular structures close to or immediately adjacent to thematerial. As used herein, close vascular structures or vascularstructures that contact, are those capillaries whose surface lies withinabout one cell layer of the surface of the material. When implantsincluding the materials of the present invention are implanted within ahost a foreign body-like capsule still forms in response to theimplantation. However, its structure is greatly altered due to the hostresponse to the material. In contrast to a standard foreign bodyresponse, a vascular bed forms at the host-material interface.

Referring now to FIG. 2, an embodiment of the present invention isillustrated. In this embodiment, a polymer membrane 30 at leastpartially surrounds an implant and includes three dimensional cavities32. At least some of the cavities 32 of the membrane 30 have asufficient size and structure to allow inflammatory cells 34 tocompletely enter therein through the apertures that define the cavities,and are defined by frames composed of strands that are less than fivemicrons in all but the longest dimension. When the inflammatory cells 34enter the cavities 32, growth of vascular structures 36 occurs withinabout one cell layer from the interface 35 of the membrane 30 and host.Although not required, vascular structures may be formed within theirregularities 32 of the membrane. Accordingly, although a foreignbody-like capsule of fibroblasts still forms that surrounds the membrane30, the entire foreign body-like capsule, including fibroblast layers,is well vascularized. The formation of close vascular structures isdependent on entry of the inflammatory cells into the cavities of themembrane so that the cells are surrounded by the strands that define theapertures and cavities. The topographic features at the implant surfacedo not effect the morphology of the inflammatory cells. Indeed,inflammatory cells at the implant surface often maintain a flatmorphology.

In selecting the size and shape of the strands and cavities 32 for thematerial 30 of the present invention, it must first be appreciated thatnot all of the cavities must have a sufficient size to allowinflammatory cells 34 to enter therein. What is required is that asufficient number of cavities 32 have a size that allows a sufficientnumber of inflammatory cells 34 to enter therein. Nor is it necessarythat all of the strands be less than five microns in all but the longestdimension. Some strands may be longer, as long as a sufficient number ofthe strands are within the prescribed size limits. The presence of asufficient number of strands and cavities of the prescribed size createsa sufficient number of vascular structures at the host-materialinterface. These vascular structures will provide sufficient nutrientsto an immunoisolated container and/or allow components and agentsproduced by cells within the interior of the chamber to enter the host.

Although at least some of the cavities 32 must have a sufficient sizeand shape to allow inflammatory cells 34 to enter therein, it is alsoimportant that extensive ingrowth of vascular and connective tissueswithin the cavities 32 does not occur. As illustrated in FIG. 3, in thecase where the apertures and cavities are too large, an extensive growthof vascular tissue 36 and connective tissue 39 occurs within a largecavity 32a; this causes the vascular tissue to be isolated within thelarge cavity. The isolation of the vascular tissue 36 within the largecavity 32a by fibroblasts and connective tissues 39 is similar to thestandard foreign body response previously discussed. By selectingcavities 32 of appropriate size, one can prevent the formation offibroblasts and connective tissue 39 therein.

It has been found that a porous polymer membrane having an averagenominal pore size of approximately 0.6 to about 20 microns and averagestrand sizes of less than about five microns in all but the longestdimension, functions satisfactorily in creating a vascular bed at thetissue-membrane interface. It should be noted, that the term "nominalpore size" is derived from methods of analysis common to the membranetrade, such as the ability of the membrane to filter particles of aparticular size, or the resistance of the membrane to the flow offluids. Because of the amorphous, random and irregular nature of most ofthese commercially available membranes, the "pore" size designation doesnot actually indicate the size or shape of the apertures and cavities,which in reality have a high degree of variability. The cavities are notreally "pores" in that they typically are not uniform regular holes orchannels through the material. Instead, these commercial membranes canbe composed of, for example, extruded filaments which act as sieves asshown, for example, in FIG. 4b. Accordingly, as used herein the term"pore size" is a manufacturer's convention used to identify a particularmembrane of a particular commercial source which has a certain bubblepoint. As used herein, the term "pore" does not describe the size of thecavities of the material used in the instant invention. The bubble pointmeasurement is described in Pharmaceutical Technology May 1983 pp. 36 to42.

As previously noted, it is not critical that all of the apertures 32(FIG. 2) of the material 30 allow inflammatory cells 34 to penetrate thematerial or, conversely prevent connective tissues from forming withinthe cavities. What is required is that a sufficient number of thecavities 32 have a size that allows inflammatory cells 34 to entertherein and yet prevent connective tissue from forming therein. In thematerials tested by Applicants the desired result is obtained where thestrands that define the apertures of the cavities have a size of lessthan about five microns in all but the longest dimension. It has beendetermined that a commercially available membrane having at leastapproximately 50% of its cavities with an average nominal size ofapproximately 0.6 to about 20 microns and strands having an average sizeof less than about five microns in all but the longest dimension willfunction satisfactorily in creating vascular structures close to themembrane.

By way of example, and not limitation, the following experiments wereperformed on commercially available membranes to determine whichmembranes result in the close vascularization of the present invention.

Numerous commercially available membranes with varying nominal poresizes were implanted in subcutaneous pockets on the backs of adult maleSprague Dawley rats for three weeks, and examined histologically. Theresults, shown in Tables 1-3, were that all membranes with apertures toosmall or having strands too closely associated to allow penetration ofmacrophages (Table 1) had standard foreign body capsules (i.e., similarto that illustrated in FIG. 1), whereas many membranes with apertureslarge enough to allow macrophages to penetrate (Table 2) had closevascular structures (i.e., similar to that illustrated in FIG. 2).

                  TABLE I                                                         ______________________________________                                        MEMBRANES THAT ARE NOT INVADED BY CELLS AND                                   DO NOT HAVE CLOSE VASCULAR STRUCTURES                                                                        Nominal                                        Company        Membrane        Pore Size                                      ______________________________________                                        Millipore      Mixed Esters Cellulose                                                                        0.1                                            Millipore      Mixed Esters Celluose                                                                         0.22                                           Millipore      Mixed Esters Celluose                                                                         0.45                                           Celenase       polypropylene   0.05                                           Celenase       polypropylene   0.075                                          Gore           PTFE/Polyester  0.02                                           Gore           PTFE/Polyester  0.2                                            Akzo           polypropylene   0.01-0.29                                      Akzo           polypropylene   0.02-0.58                                      Akzo           polyethylene    0.1                                            Akzo           polyethylene    0.08                                           Akzo           polyethylene    0.6                                            Supor          polysulfone     0.1                                            Amicon         YC, YM, PM, XM  10-300 kD                                      Omega          polyethersulfone                                                                              100-30okD                                      Millipore      Durapore ®  0.22                                           Millipore      Immobilon-n ®                                                                             0.22                                           Gelman         Versapore ® 0.22                                           Gelman         Supor ®     0.22                                           Gelman         Supor ®     0.8                                            Gelman         Polysulfone HT-200                                                                            0.22                                           Gelman         Polysulfone HT-200                                                                            0.6                                            Gelman         Polyester       0.22                                           Gelman         Polysulfone/polyester                                                                         0.8                                            Sartorius      Cellulose Acetate                                                                             0.22                                           Sartorius      Cellulose Acetate                                                                             0.22                                           Sartorius      Cellulose Acetate                                                                             0.45                                           Sartorius      Cellulose Acetate                                                                             0.65                                           Sartorius      Cellulose Nitrate                                                                             0.22                                           Sartorius      Reinforced Cell. Acet.                                                                        0.22                                           Nucleopore     Polyester       0.8                                            Pall           Uncharged Nylon 0.22                                           AMF Cumo       Charged Nylon   0.22                                           Micron Separation                                                                            Nylon 66        0.22                                           Inc.                                                                          Micro Filtration                                                                             Cellulose Acetate                                                                             0.22                                           Sys.                                                                          Micro Filtration                                                                             Cellulose Acetate                                                                             0.22                                           Sys.                                                                          Akzo           Polypropylene-HF                                                                              0.2-0.8                                        ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        MEMBRANES THAT ARE INVADED BY CELLS                                           AND HAVE CLOSE VASCULAR STRUCTURES                                                                       Nominal                                            Company      Membrane      Pore Size                                          ______________________________________                                        Millipore    Mixed Esters Cellulose                                                                      1.2                                                Millipore    Mixed Esters Cellulose                                                                      8.0                                                Sartorius    Cellulose Acetate                                                                           0.8                                                Sartorius    Cellulose Acetate                                                                           1.2                                                Sartorius    Cellulose Acetate                                                                           3.0                                                Sartorius    Cellulose Acetate                                                                           5.0                                                Sartorius    Cellulose Acetate                                                                           8.0                                                Gore         PTFE/Polyester                                                                              1.0                                                Gore         PTFE/Polypropylene                                                                          3.0                                                Gore         PTFE/Polyester                                                                              3.0                                                Gelman       Versapore ®                                                                             0.8                                                Gelman       Versapore ®                                                                             1.2                                                Gelman       Versapore ®                                                                             3.0                                                Gelman       Versapore ®                                                                             5.0                                                ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        MEMBRANES THAT ARE INVADED BY CELLS                                           BUT DO NOT HAVE CLOSE VASCULAR STRUCTURES                                                               Nominal                                             Company        Membrane   Pore Size                                           ______________________________________                                        Tetco          Polyester  3                                                   Tetco          Polyester  5                                                   Tetco          Polyester  8                                                   Tetco          Nylon      10                                                  Tetco          Nylon      10                                                  Tetco          Nylon      10                                                  Millipore      PTFE       5                                                   Millipore      PTFE       10                                                  Nucleopore     Polycarbonate                                                                            1                                                   Nucleopore     Polycarbonate                                                                            3                                                   Nucleopore     Polycarbonate                                                                            8                                                   Nucleopore     Polycarbonate                                                                            12                                                  ______________________________________                                    

For example, membranes created from mixed esters of cellulose and havingnominal pore sizes of 0.1, 0.22, and 0.45 microns did not induce closevascular structures when subcutaneously implanted into rats. However,mixed esters of cellulose membranes with nominal pore sizes of 1.2 and 8microns did induce close vascular structures. Similarly, celluloseacetate membranes having a nominal pore size of 0.2, 0.45, and 0.65microns and teflon membranes having a nominal pore size of 0.02 and 0.2microns did not induce close vascular structures. But, cellulose acetatemembranes having a nominal pore size of 0.8, 1.2, 3, 5, and 8 microns,and teflon membranes having a nominal pore size of 1.0 and 3.0 micronsdid induce close vascular structures.

In membranes wherein close vascular structures were seen, the membranewas penetrated by inflammatory cells from the host. It is believed thatthe formation of close vascular structures is related to cellularinvasion. However, numerous membranes that did allow penetration ofinflammatory cells did not have close vascular structures (Table 3),indicating that invasion by inflammatory cells was perhaps related, but,not necessarily sufficient for the production of close vascularstructures.

Scanning Electron Microscope (SEM) analysis of the membranes revealedthree dimensional structural or architectural properties thatdistinguish membranes that do have close vascular structures (positiveresponse) from those that do not (negative response). Membranes with apositive response had high porosities and were composed of strands(fibers, filaments, microglobules, cone-like or rod-like structures witha small diameter (<5 microns)). For example, Millipore brand mixedesters of cellulose membranes with nominal pore size of 5 μm arecomposed of irregular, amorphous globular structures and strands withdiameters from about 1 to 3 μm, and irregular cavities from 0.5 to 5microns in diameter, and having a percent porosity of 75% (FIG. 4a).Gore® teflon membranes with a nominal pore size of 3 μm are composed ofstrands with diameters of less than about 1 micron that interconnectwith teflon clusters less than about 3 microns in diameter (FIG. 4b).The cavities are very elongated being generally about 1 to 2 micronswide by 10 to 15 μm long. After implantation, both of these membraneswere invaded by inflammatory cells which had a round morphology underthe light microscope (see invading cells in FIG. 2), and bothconsistently had close vascular structures.

In contrast, membranes with a negative response had apertures andcavities defined by strands with a relatively high surface area, largeenough for inflammatory cells to use as a substrate to flatten against.For example, Millipore brand teflon membranes with a nominal pore sizeof 5 microns (FIG. 5a) are composed of globular or plate-like structuresabout 5 to 10 microns in diameter, and have irregular amorphous cavitiesabout 5 to 10 microns in diameter. Nuclepore brand membranes with anominal pore size of 12 microns (FIG. 5b), have uniform circular holesthat are 9 microns in diameter that are scattered within a membranesheet, with from 5 to 25 microns between the edges of the holes. Afterimplantation, both of these membranes were invaded by cells but thecells maintained a flattened morphology (see invading cells in FIG. 6).

Thus, the three dimensional conformation or architecture of thestructures that delimit the cavities and irregularities have a stronginfluence on the biological response.

Applicants have further determined that materials with a positiveresponse had structural features that caused penetrating cells to assumea round morphology. Whereas materials with a negative response hadstructural features that caused penetrating cells to assume a relativelyflattened morphology.

Membranes with a negative response have a standard foreign body capsuleafter implantation, and were invaded by inflammatory cells that assumedan elongated, highly flattened morphology (FIG. 6). FIG. 6 is a lightmicrograph illustrating a teflon membrane (the same membrane illustratedin FIG. 5a) implanted for 3 weeks in a subcutaneous dorsal pocket in arat. Note the extensive cytoplasm of the cells invading thepolytetrafluoroethylene ("PTFE") membrane shown in FIG. 6. The cellsappear to have flattened against the plate-like PTFE structure and havethe appearance of cells of a standard foreign body response (FIG. 1) incontrast to the rounded cells invading the membrane in FIG. 2.

This is consistent with the observation of rounded mononuclear cellsinvading an implant during the early, acute phase of a foreign bodyresponse, followed by flattened cells on the surface of implants in thelater, chronic standard foreign body response to implants with a smoothsurface (e.g., FIG. 1). The flattening of the macrophages and foreignbody giant cells against the surface walls off the implant, is followedby a quiescent, chronic response characterized by a lack of new invadingmononuclear cells and a lack of new vascular growth in the periphery ofthe foreign body. Macrophages and foreign body giant cells from theinitial host reaction to the implant remain, but are generally flattenedagainst the foreign material. This is a long-term response that resultsin a permanent walling off of implants that are non-digestible by themacrophage. The maintenance of a long-term foreign body response ischaracterized by inflammatory cells which spread upon and cover theforeign material. Applicants have discovered that this response appearsto require a surface-like area capable of acting as a substrate forflattening and spreading of the cells.

When the implanted material has an architecture of strands that have adiameter (<5 μm) too small or configuration too irregular to allow asurface for flattening of cells, as do the membranes that give apositive vascular response (FIG. 2 and Table 2), the efforts of theinflammatory cells to cover and wall off the material are thwarted, andthe cells do not obtain a flattened morphology. Instead, they remainrounded and Applicants hypothesize that the inflammatory cells inducethe formation of close vascular structures at the material-hostinterface. The implanted material is never completely walled off, andtherefore a chronic response is never obtained.

Flattening and activation of inflammatory cells (which leads to foreignbody capsule formation) is observed where the implant material providesa structure onto which the inflammatory cells can flatten and spread. Aninflammatory cell does not require a smooth area for flattening. Forexample, an area composed of closely adjacent pillars of equal heightand diameter might be recognized by the inflammatory cell as essentially"smooth" and the cells would then spread on the surface.

Applicants further hypothesize that if the inflammatory cell nucleuscannot enter a cavity or irregularity then the cell will "see" thematerial as flat and will flatten onto the material at that location.Conversely, cells in contact with a cavity or irregularity from morethan one direction or plane will not "see" a flat area and will retain arounded conformation or even conform to the shape of the cavity orirregularity. Accordingly, material having a surface-like area greaterthan about 5 microns would not be likely to result in closevascularization. For example, the material shown in FIG. 5a which gave anegative response has many cavities and irregularities which are smallerthan about 6 um, but it also has leafy-appearing somewhat flatstructures onto which macrophage may flatten. Accordingly, in thepresent invention material must be selected so that it has sufficientirregularities and cavities to prevent substantial numbers ofinflammatory cells from flattening. The rounded cell may conform to thecavities and irregularities but will not flatten. Formation of someflattened cells, especially at the "surface" of the implant is oftenseen and is within the scope of the invention provided that there arenot so many flattened cells that the material is walled off bynonvascularized fibroblasts.

Macrophage behavior is not yet fully understood. It is believed thatmacrophages are activated when they become flat. Upon activation theyare believed to secret factors which signal fibroblasts to form andproliferate. Accordingly, Applicants hypothesize that by utilizing amaterial whose three dimensional cavities and irregularities prevent themacrophage from flattening, this invention will avoid macrophageactivation and consequent formation of the typical foreign body capsule.On the other hand, it may be that rounded macrophages are secretingfactors that either stimulate neovascularization directly or interruptan existing suppression of new vascularization.

The host inflammatory cell response described above for the variousmaterials is generally observed for up to about 12 weeks followingimplantation. Thereafter, in both the standard foreign body capsuleresponse and in the use of the instant invention, the inflammatory cellsgradually diminish leaving either a stable foreign body capsule or, inthe instant invention, a stable vascularized bed. The Applicants haveobserved a stable vascular bed for 1.5 years in subcutaneous implants of3 μm Gore™ teflon in rats.

When the material utilized has the three dimensional architecture setforth above, a vascularized membrane is achieved. To this end, theendothelial cells that make up the capillary walls are immediatelyadjacent to or very close to the material-host interface. There are no,or few, intervening macrophages or fibroblasts. Accordingly, moleculescoming through the material will be at the surface of an endothelialcell for transportation into the capillaries. For example, moleculessecreted by pancreatic islet cells on one side of the material will beavailable for uptake by capillaries on the other side of the material.Likewise, molecules such as glucose coming from the capillary, will besensed by islet cells contained within an implanted chamber made of thematerial. The resistance to diffusion of such molecules will be relatedto the distance necessary to traverse the material.

Applicants tests of commercially available membranes (Tables 1-3)indicate that close vascular structures will likely result with amaterial having an average nominal pore size in the range ofapproximately 0.6 to about 20 microns and being composed of strands,fibers, cones, rods, or microglobules with a diameter no greater thanapproximately 5 microns.

Additional tests have shown that when the average aperture size isgreater than approximately 40 microns, although vascular structures growinto the cavities of the membranes the capillaries are not in contactwith or adjacent to the material but rather typically lie at somedistance from the material due to a halo of macrophages and fibroblastsin a connective tissue matrix that surrounds the capillaries asillustrated in FIG. 3. Thus, as in the case of a foreign body capsule onthe surface of a membrane, the capillaries are separated from thepolymer surface by several layers of cells producing the same kind ofdiffusive resistance encountered in a classical foreign body response.

In contrast to the present invention, in a typical implant, the implantis encapsulated by the foreign body capsule and is typically at theedges of a large cellular vascular space, see FIG. 1.

The close vascularization of the present invention improves on previousbiopolymer implants because the vascular bed is formed immediatelyadjacent to the material-host interface. As set forth in more detailbelow, this method of vascularization has a variety of applications. Forexample, the material can be used in conjunction with an indwellingsensor, an indwelling catheter, and for an immunoisolation container.

Referring now to FIG. 7, an immunoisolation membrane 42 is illustrated.As illustrated, the membrane 42 is selected such that it allowsmacrophages 34 to enter at least some of the cavities 44 of the membranecausing vascular structures 46 to be formed at the host-membraneinterface 47. Again, it should be noted that although some vascularstructures can be formed within the cavities 44 of the membrane 42, thisis not critical to the success of the material or the creation of avascular bed.

As illustrated, the membrane 42 surrounds at least a portion of a secondmembrane or layer 50 that defines an immunoisolated interior 52. Thisinterior 52 can include tissue 54 that must be protected from contact byhost cells that would reject the implanted cells. For example,allografts or xenografts or in the case of isografts, such as autologousimplants of genetically engineered cells, the membrane would need onlyto prevent passage of cells to prevent movement of the geneticallyengineered cells, which often contain retroviral vectors, out of themembrane enclosures and into the host tissues. This isolation of grafttissues from host tissue represents a significant advance over previousmethods used for autologous transplantation of genetically engineeredcells, because it prevents the genetically engineered cells frompotentially invading host tissues in an unregulated manner and causingtumors in the host via the retroviral vector.

On the other hand, it is desirable that the second membrane 50 allow forthe diffusion of components generated by the tissues 54, for example,insulin from pancreatic islets. Likewise, it is desirable that thesecond membrane 50 allow nutrients from the host to enter the interior52 of the implant and nourish the tissue 54. To this end, the secondmembrane 50 preferably includes pores 56 that allow glucose or othercomponents to diffuse into the first membrane 42 but preventsmacrophages 34 and/or humoral factors from entering the second membrane.

Although the device illustrated in FIG. 7 includes two membrane layers,it should be noted that other constructions can be utilized. Forexample, referring to FIG. 8, the device includes a single membrane 61that includes cavities 62 having a gradient of size. The larger outercavities 62 allow macrophages to enter at least an outer portion 64 ofthe cavity 62, causing vascularization at the host-membrane interface65. However, the smaller inner cavities 66 prevent macrophages fromentering an inner portion of the membrane and thereby isolating aninterior 68 defined by the membrane.

Referring now to FIG. 9, an indwelling catheter 70 including anembodiment of the material 72 of the present invention is illustrated.Such a catheter 70 can be, for example, a catheter for continuousambulatory peritoneal dialysis.

As illustrated, the material 72 covers the wall 74 of the catheter 70and allows the creation of a vascular bed around the catheter 70. Thewall 74 of the catheter 70 is preferably impenetrable to both cells andmolecules.

In typical catheter designs, a thick foreign body of nonvascularizedcollagenous material is produced around the catheter that acts as aconduit for bacteria. In the present invention, vascularization aroundthe catheter prevents tunnel site infections because necrosis of thetissue is prevented and the vascular bed bathes the area with the entirerepertoire of blood borne immune cells. In another embodiment, a flangeon a catheter would be covered with a vascularizing material, or wouldbe made entirely from the material.

Referring now to FIG. 10, a sensor 80 including an embodiment of thematerial 82 of the present invention is illustrated. Such a sensor 80can include, for example, a glucose sensor for monitoring glucose levelsin diabetics. As illustrated, the material 82 covers a body 84 whichcontains an electrode 85 of the sensor 80 and causes a vascular bed 86to be created around the sensor 80. The creation of the vascular bedcircumvents the problem of foreign body occlusion typically encounteredwith indwelling sensors.

The vascular response is believed to be unrelated to the composition ofthe material. This is illustrated by the above examples wherein similarresponses of the tissue were found with respect to hydrophilic(cellulose) and hydrophobic (teflon) materials. Therefore, the inventorsbelieve that the material can be constructed from a variety of polymersincluding, inter alia, polyethylene, polypropylene, teflon, celluloseacetate, cellulose nitrate, polycarbonate, polyester, nylon, polyester,polysulfone, mixed esters of cellulose polyvinylidene difluoride,silicone, and polyacrylonitrile. Known biocompatible medical implantsare composed of ceramics and metals. Assuming these materials could bemanipulated to provide the three dimensional structures describedherein, they would also be useful in the present invention.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present invention andwithout diminishing its attendant advantages. It is therefore intendedthat such changes and modifications be covered by the appended claims.

We claim:
 1. A device having a wall defining a chamber for holding living cells for implanting into a mammalian host wherein the wall comprises:(a) a first zone of a first porous material proximate the chamber wherein the first porous material is permeable to the flow of nutrients from the host to living cells and products from the living cells to host and impermeable to host immune cells and maintains immunoisolation of the chamber; and (b) a second zone of a second porous material outside of the first zone comprising the porous structure as shown in FIG. 4(a) or 4(b), wherein the second porous material comprises frame of elongated strands of material that are less than 5 mm in all but the longest dimension, said frames define apertures which interconnect to form a three dimensional cavities which permit substantially all inflammatory cells migrating into the cavities to maintain a rounded morphology and which promote host vascularization adjacent to but not substantially into the second zone upon implantation into a host.
 2. The device of claim 1, wherein the second porous material has a nominal pore size ranging from about 0.6 to 20 μm.
 3. The device of claim 1, wherein the first porous material is impermeable to host immune factors.
 4. The device of claim 1, wherein the second porous material is selected from the group consisting of mixed esters cellulose having a nominal pore size ranging from 1.2 to 8.0 μm; cellulose acetate having a nominal pore size ranging from 0.8 to 8.0 μm; and PTFE/polyester having a nominal pore size ranging from 1.0 to 15 μm. 